Asteroid Day… and what may follow

The Tunguska explosion in 1908 was due to the arrival of a small (perhaps 50 metre) cosmic object, quite likely a fragment of a known comet. Astronomers are now wondering whether siblings of that projectile might pass close by the Earth over the next week or so.

I write tonight on Asteroid Day, which occurs on June 30th each year: the anniversary of the Tunguska event in 1908. All around the globe there are meetings, talks, TV and radio shows, and newspaper articles, discussing the hazard that asteroids and comets pose to our civilisations. All around the globe, perhaps, but with little happening in New Zealand.

It happens that the Tunguska explosion is an event about which I have written several reports, and published a handful of journal papers in trying to assist our understanding of what occurred, and what it means. On the centenary, in 2008, I prepared a discussion for Nature covering the various ideas put forward over the decades for the cause of the huge explosion. In 1998, with my erstwhile colleague David Asher, I published a paper in the journal Planetary & Space Science concerning the dynamical evolution of the peculiar comet 2P/Encke, and how the Tunguska projectile might well have been a fragment from a break-up of that comet over the past 20-30,000 years. This is a broad subject about which we wrote a series of papers, another example from 1996 being this one, or this one in 1995.

In 1995 I also pointed out that on the same day as the Tunguska arrival, a meteorite fell elsewhere in Russia (at Kagarlyk), and might be genetically related to the larger object.

A radically-different way of addressing the Tunguska event was an investigation of whether Douglas Mawson, at the time in the Antarctic, might have witnessed an exceptional aurora correlated with the explosion. This study was prompted by claims by several Russian authors that Mawson (or, as they wrote, ‘Mouson’) could have been reporting something similar to the artificial auroras apparently produced by large atmospheric nuclear tests during the 1950s at the antipodal geomagnetic locations. (The Tunguska explosion occurred at an altitude near 10 km, and involved an energy release variously estimated as being between 3 and 20 megatonnes of TNT equivalent. For comparison, the Hiroshima atomic bomb yield was about 13 kilotonnes, and the most-powerful hydrogen bomb ever tested released energy equivalent to 60 megatonnes of TNT.)

The above is all fairly routine, in terms of small asteroids arriving in the atmosphere and making a rather large bang. The last such event to cause any physical damage to people was in 2013, near Chelyabinsk in Russia. Other similar atmospheric explosions seem to have occurred during the 20th century, as I wrote a couple of decades ago. Things do go bang in the night; and sometimes during the day. Indeed only a week or so ago a small meteoroid/asteroid, around three metres in size, was detected prior to arrival in the atmosphere, its disintegration resulting in the release of energy equivalent to about six kilotonnes of TNT in a huge flash over the sea just south of Jamaica. If you’d like to see more details, look here.

A rather different thing is this… Meteor showers occur when Earth passes through a meteoroid stream produced by the decay of a comet or asteroid. For a few days, maybe a week, we observe the shower either in the night sky, or perhaps on the day side by radar. But is there any reason to think that only small meteoroids exist in streams? Typical shooting stars (visible meteors) are around pea- to marble-sizes; the far fainter ones we can detect by radar are close to sand-grain size. Perhaps, though, larger lumps might lurk there. Indeed, we saw essentially this in mid-1994, when Comet Shoemaker-Levy-9‘s ‘string of pearls’ zapped into Jupiter.

Rather than getting hit by 21 cometary fragments in a week or so, as was Jupiter in that episode, a periodic comet in the inner solar system might break apart – we actually see comets doing this all the time – and leave a trail of detritus in sizes ranging from dust to kilometre-sized lumps. Most years we might simply see meteor showers, from smaller pieces entering the atmosphere. (Note that typically the Earth is struck by 100 tonnes of meteoroids and dust each day, or 40,000 tonnes a year.) Every so often, however, the clockwork of the heavens could bring around a concentration of such larger blocks. Then there might be chaos.

Comet 2P/Encke, indeed, has been known for more than 70 years to be associated with a substantial number of meteor showers, some night time (produced by the part of the stream orbit coming inwards towards the Sun to cross Earth’s orbit), but also day-side showers caused by debris moving on the orbital leg passing away from the Sun in the gigantic elliptical loop that constitutes a meteoroid stream. Matter of fact, the dynamical evolution is far more complicated than the above few sentences can convey, and I have spent much of my professional life investigating how the orbits evolve in time; but I hope I have given you the gist of things.

We have known since the late 1940s that there are daytime meteor showers that are part of this complex stream spawned by Comet Encke, the two main showers being termed the Beta Taurids and the Zeta Perseids. In my opinion, the Tunguska object was simply a large fragment in the Beta Taurid shower. Its observed speed and arrival direction are consistent with this. In fact it was this matter that I wrote about in a paper in 1988 that caught the eye of Arthur C. Clarke, leading to him writing to me, from which start we began a collaboration.

I have written extensively about the asteroid and comet impact hazard, in hundreds of newspaper and magazine articles, but also the two books above published in 1995 (Rogue Asteroids and Doomsday Comets) and 2000 (Target Earth).

Is this all moot, simply a matter of scholarly opinion? Well, it happens that dynamical modelling such as that I alluded to above leads to an expectation that in some years the Beta Taurid or Zeta Perseid showers might be more intense than others, and at such times larger fragments in groups might come close by the Earth. The last time this happened was in 1975, when a number of lunar impacts were picked up by Apollo seismometers. And the next time the clockwork of the heavens works against us? Well, this year.

A paper by researchers at the University of Western Ontario in Canada (where I spent some months working on this in 2014) suggests this to be the case. As of yet their paper is yet to be published, but is available as a pre-print; and here it is.

Whilst nothing much has been said about this in the NZ media, elsewhere there have been quite a few articles that have appeared. In case you are interested, here are links to just a handful.

Duncan Steel is a space scientist based in Nelson. He has worked in space research for over forty years, with times spent with NASA, ESA, various universities and observatories, and also running his own company. Duncan is the author of four books, over a hundred research papers, and more than a thousand articles for newspapers and magazines. He has also appeared in hundreds of radio and TV programmes. Minor planet/asteroid (4713) Steel is named for him, as is a lunar-roving robot in one of Arthur C. Clarke's SciFi novels.

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